Yarn withdrawal nozzles for open-end rotor spinning devices are known in various embodiments.
There is a well-known problem in connection with open-end spinning devices, in that the real yarn twist initiated by the rotation of the spinning rotor does not continue evenly into the yarn end being created, that is, the real yarn twist is applied in an intensified manner to the extent of yarn which is located between the yarn withdrawal nozzle and the yarn withdrawal device of the open-end rotor spinning device. However, the real yarn twist often continues quite incompletely into the yarn extent located upstream of the yarn withdrawal nozzle.
Since too low a yarn twist in the yarn extent located between the fiber collection groove and the yarn withdrawal nozzle has an extremely negative effect on the spinning stability of an open-end rotor spinning device, yarn withdrawal nozzles which are provided with a special surface structure, preferably roughened, have already been developed in the past. Such rough surface structures are intended to increase the friction between the withdrawal nozzle surface and the rotating yarn, and thereby to apply a false twist in addition to the real yarn twist to the yarn extent being created. Since the false twist being created extends into the yarn extent located between the fiber collection groove and the yarn withdrawal nozzle, such a surface structure of the yarn withdrawal nozzle leads to an improvement of the spinning stability of the open-end spinning device.
Different embodiments are known in regard to the surface structure of yarn withdrawal nozzles. German Patent Publications DE-OS 25 44 721 and DE 33 44 741 A1 or European Patent Publication EP 0 422 615 B1, for example, describe yarn withdrawal nozzles having notch-like depressions in the area of the yarn inlet zone.
In accordance with DE-OS 25 44 721, the yarn withdrawal nozzle is made of an oxide-ceramic material and has a peak-to-valley height of 0.2 to 0.7 Tm in the area of the yarn inlet funnel. Notches, which can have various notch opening angles, are furthermore arranged in the area of the yarn inlet funnel.
The yarn withdrawal nozzles described in DE 33 44 741 A1 have an exchangeable yarn inlet funnel, which has been drawn from sheet steel and subsequently hardened. Here, the yarn inlet funnel either has notches or protruding beads. It is furthermore known from this patent to position notches in two ring-shaped arrangements located one behind the other in the direction of yarn travel.
EP 0 422 615 B1 relates to a yarn withdrawal nozzle which has notches in the area of the yarn inlet zone and protrusions in the area of the yarn withdrawal conduit. Here, the notches are intended to improve the spinning stability of the open-end spinning device, while the protrusions in the yarn withdrawal conduit are used to produce an especially napped yarn.
Although it has been possible by means of the above described yarn withdrawal nozzles, known as "notched nozzles", to increase the spinning stability of the open-end spinning devices, the improvement of spinning stability occurred, at least in part, at the expense of the yarn quality which could be achieved.
In connection with yarn withdrawal nozzles for open-end spinning devices it has furthermore been long known to arrange a spiral structure in the area of the yarn inlet zone in place of notches. Swiss Patent 503 127, German Patent Publications DE 37 07 256 A1 and DE 42 24 632 A1, Japanese Utility Model Sho 51-130 829 or European Patent Publication EP 0 220 546 A1 relate to yarn withdrawal nozzles designed in this manner.
In such yarn withdrawal nozzles, the spiral structure can be embodied as a strip-shaped raised part or as a groove-shaped depression, as explained in German DE 37 07 526 A1 and Swiss Patent 503 127, and preferably extends over the entire yarn inlet zone of the yarn withdrawal nozzle up to the start of the yarn withdrawal conduit.
Japanese Utility Model Sho 51-130 829 discloses different variations of yarn withdrawal nozzles with a spiral surface structure. In connection with one of the embodiments represented, the spiral structure extends past the area of the yarn inlet zone and into the yarn withdrawal conduit.
An open-end spinning device with a yarn withdrawal nozzle with a spiral bead in the area of the yarn inlet zone is described in European Patent Publication EP 0 220 546 A1. A tube with beads lying obliquely in respect to the yarn traveling direction is connected downstream of the yarn withdrawal nozzle.
A comparable yarn withdrawal nozzle with a spiral structure is also described in German DE 42 24 632 A1. This known "spiral nozzle" has surfaces which extend in the manner of a spiral and are linear in respect to the yarn traveling direction. To obtain yarn reversing points, the surfaces have been inclined at an angle in respect to each other.
With such an embodiment, the yarn is supported over relatively large areas on the surfaces which are linear in respect to the yarn traveling direction, and in this way the surface pressure acting on the yarn is reduced to a permissible amount. Furthermore, by means of the yarn transition points formed between the linear surfaces at an angle to each other, thrust components are transferred to the yarn which act to keep the yarn twist in the area between the fiber collection groove and the yarn withdrawal nozzle at a higher value.
As a rule, with the above described yarn withdrawal nozzles, known as "spiral nozzles", the yarn slides over raised parts of the spiral structure in the course of being withdrawn. In this case the orientation of the spiral structure is such that a thrust component at the yarn extent being created in the direction toward the fiber collection groove becomes effective with an appropriate rotation direction of the yarn. This thrust component has the effect that the real yarn twist introduced between the fiber collection groove and the yarn withdrawal nozzle is at least partially prevented from leaving this area.
In contrast to withdrawal nozzles with a smooth surface, it is possible by means of such "spiral nozzles" to introduce a higher twist into the yarn extent ahead of the yarn withdrawal nozzle, and in this manner to lower the number of occurring yarn breaks. Although as a rule "spiral nozzles" achieve better yarn qualities in comparison with the so-called "notched nozzles", they have the disadvantage that the spinning stability which can be achieved is often unsatisfactory.